The present invention relates to processes for creating and controlling the crystal texture of thin films with anisotropic ferroelectric polarization or permittivity and to the integrated circuits which include them.
Ferroelectrics are analogous to ferromagnetic materials: just as the ferromagnetic material in a bar magnet can be permanently magnetized by applying a sufficiently strong magnetic field to it, and will thereafter act independently as a magnet, so a ferroelectric can acquire a fixed voltage gradient when a sufficiently strong electric field is applied to it.
Bismuth (Bi) containing ferroelectrics, such as SrBi2Ta2O9 (SBT), have attracted considerable attention for use in nonvolatile memories because of their intrinsic low operating field, high switching speed, and excellent endurance. Unlike ferroelectric Pb(Zr,Ti)O3 (PZT), a leading alternative, SBT is essentially free of fatigue (decrease in efficiency) to at least 10 exp 11 cycles even when Pt electrodes are employed. This eliminates the need for conducting oxide electrodes, simplifying capacitor fabrication. Moreover, SBT generally exhibits a smaller coercive voltage then PZT. For these reasons, SBT is a leading contender for the storage cell material in nonvolatile memory devices.
However, SBT (see FIG. 7 for a diagram of an SBT crystal) is one of a general class of bismuth containing ferroelectrics known as Aurivillius compounds. Aurivillius compounds are characterized by a layered perovskite unit cell. A perovskite crystal is the simplest example of a ferroelectric crystal. In a perovskite crystal, not every ion is at a point of full cubic symmetry. (See FIG. 6 for an example of a perovskite structure. Note that every Ba++ and Ti4+ ion is at a point of full cubic symmetry, but that the Oxe2x88x92 ions are not.) The Aurivillius phases can be represented by the general formula (Bi2O2)2+(Amxe2x88x921BmO3m+1)2xe2x88x92, where A can be monovalent, divalent, or trivalent ions present singly or in combination. Examples for A include Na+, K+, Sr2+, Ca2+, Ba2+, Pb2+, Bi3+, etc. The B ion is Ta5+, Ti4+, Nb5+, Mo6+, W6+, Fe3+, etc. or a mixture of them. The quantity m can be an integer or half integer. (See E. C. Subbarao, xe2x80x9cA Family of Ferroelectric Bismuth Compounds,xe2x80x9d 23 J. Phys. Chem. Solids 665 (1962); E. C. Subbarao, xe2x80x9cFerroelectricity in Mixed Bismuth Oxides with Layer-Type Structure,xe2x80x9d 34 J. Chem. Phys. 695 (1961); E. C. Subbarao, xe2x80x9cFerroelectricity in Bi4Ti3O12 and Its Solid Solutions,xe2x80x9d 122 Phys. Rev. 804 (1961); and B. Aurivillius and P. H. Fang, xe2x80x9cFerroelectricity in the Compound Ba2Bi4Ti5O18,xe2x80x9d 126 Phys. Rev. 893 (1962) which are hereby incorporated by reference.) Other ferroelectrics in this family include SrBi4Ti4O15, CaBi2Nb2O9, Bi4Ti13O12, and solid solutions such as SrBi2(TaxNb2xe2x88x92x)O9.
In general, Bi-layered ferroelectrics possess a large polarization along the a-axis, but virtually no polarization along the c-axis, meaning that they have highly anisotropic properties. (An xe2x80x9canisotropicxe2x80x9d property of a material is one which depends on the orientation of the material. For example, wood is anisotropic, in that it splits more easily with the grain than across the grain.) Therefore, the ferroelectric properties (spontaneous polarization, coercive field, dielectric constant, etc.) are strongly dependent on the orientation of the films with respect to the underlying substrate materials. For bulk compounds, the major axes are fairly simple to characterize since crystal sections can be cut in such a way as to facilitate the application of high fields along different crystallographic directions. However, the properties in bulk compounds can never be truly replicated in the corresponding thin films. To date, the majority of SBT thin film work indicates a strong tendency towards polycrystalline or c-axis oriented growth, regardless of the electrode employed. Deposition of a-axis oriented SBT has proven to be quite difficult due to the strongly flattened unit cell characteristic of the layered perovskites (for SBT, a=0.38 nm and c=2.51 nm). In order to maximize the ferroelectric properties of SBT thin films, methods must be developed to obtain highly a-axis textured layers.
Aurivillius compounds are not the only ferroelectrics exhibiting anisotropic polarization. Polarization is also anisotropic in ferroelectrics possessing the perovskite structure and the tungsten-bronze structure. Examples of the perovskite structure include BaTiO3, PbTiO3, Pb(Zr,Ti)O3, lanthanum-doped Pb(Zr,Ti)O3, and niobium-doped Pb(Zr,Ti)O3. FIG. 6 depicts a diagram of the crystal structure of BaTiO3. Some examples of the tungsten-bronze compounds are PbNb2O6, PbTa2O6, SrNb2O6, and BaNb2O6. The most technologically significant example of these compounds, however, is SrxBa1xe2x88x92xNb2O6.
Therefore, since perovskite and tungsten-bronze structures are anisotropic, a method must also be developed for obtaining perovskite and tungsten-bronze films that are oriented along the direction of maximum polarization. For perovskite materials, the desired orientation depends upon the structure of the film, which depends upon the composition chosen. For compositions within the tetragonal phase, the largest ferroelectric polarization occurs along the c-axis direction. For compositions within the rhombohedral phase, the maximum ferroelectric polarization occurs along the [111] direction. (See C. M. Foster, G. R. Bai, Z. Li, R. Jammy, L. A. Wills, and R. Hiskes, xe2x80x9cProperties Variation with Composition of Single-Crystal Pb(ZrxTi1xe2x88x92x)O3 Thin Films Prepared by MOCVD,xe2x80x9d 401 Mat. Res. Soc. Symp. Proc. 139 (1996); D. J. Wouters, G. Willems, E. G. Lee, and H. E. Maes, xe2x80x9cElucidation of the Switching Processes in Tetragonal PZT by Hysteresis Loop and Impedance Analysis,xe2x80x9d 15 Integrated Ferroelectrics 79 (1997); and K. G. Brooks, R. D. Klissurska, P. Moeckli, and N. Setter, xe2x80x9cInfluence of Texture on the Switching Behavior of Pb(Zr0.70Ti0.30)O3 Sol-Gel Derived Thin Films,xe2x80x9d 12 J. Mater. Res. 531 (1997) which are hereby incorporated by reference.)
Ferroelectrics are not the only materials exhibiting anisotropic crystal structures. Many dielectrics also exhibit anisotropic crystal structures resulting in highly anisotropic permittivities. Materials with high permittivities are useful for standard capacitor elements: a capacitor with a given capacitance can be constructed with a smaller electrode plate area, by using a dielectric with a high permittivity rather than one with a lower permittivity. Therefore, orienting the dielectric film such that the permittivity is maximized is very important.
One method of texture control is ion-bombardment. Accelerated ions disturb solids within a shallow layer defined approximately by the depth of their penetration. Therefore ion beams may induce drastic changes in the structure of thin solid films which could acquire new physical properties.
It has been shown that ion bombardment during thin film growth can modify the structure, chemistry, and physical properties of the layers as they are deposited. (See F. Adibi et al., xe2x80x9cEffects of High-Flux Low-Energy (20-100 ev) Ion Irradiation During Deposition on the Microstructure and Preferred Orientation of Ti0.5A10.5N Alloys Grown by Ultra High-Vacuum Reactive Magnetron Sputtering,xe2x80x9d 73 J. Appl. Phys. 8580, (1993) which is hereby incorporated by reference.)
Innovative Structures and Methods
The present application discloses a method that allows for precise control of the crystallographic orientation of ferroelectric thin films on a variety of electrodes, resulting in optimal ferroelectric properties by use of ion bombardment to nucleate growth of the proper oriented layer. For maximum polarization, an Aurivillius compound, such as SBT, must be oriented such that the a-axis points at a right angle away from the surface of the film as shown in FIG. 8a and a perovskite compound, such as PZT, must be oriented such that the c-axis points at a right angle away from the surface of the film as shown in FIG. 8b. In a specific example, SBT thin films have been grown with an a-axis orientation as shown in FIG. 8a using the disclosed method.
The present application also discloses a method that allows for the precise control of the crystallographic orientation of dielectric thin films on a variety of electrodes, resulting in films exhibiting the largest possible permittivity. Applications which would benefit from these films include dynamic random access memories (DRAMs), and on-chip capacitors for RF devices. Examples of on-chip capacitors for RF applications include bypass, decoupling, tunable, and switched filter capacitors.